1. Field of Invention
The present invention relates generally to monitoring impairments in a Hybrid Fiber-Coax (HFC) network, and more particularly to apparatus and methods for detecting and locating linear impairments such as, but not limited to, impedance mismatches causing micro-reflections in a coaxial cable plant of an HFC network.
2. Background Art
The detection and location of different linear distortions (e.g., from impedance mismatches) in a modern HFC network that carries digital signals is a challenging problem. Traditional methods of detection and location of such impairments have involved Time-Domain Reflectometry (TDR). In general, TDR involves the injection of a high power probe signal or pulse into the cable (“active TDR”) and the measurement of the time delay, level and polarity of the echo signal reflected from different mismatches. However, in a coaxial cable plant of an HFC network, the use of active TDR becomes tricky because of the presence of a wideband downstream spectrum of service signals, including analog TV channels, digital QAM channels, digital OFDM signals, AGC pilots, and the like. The probe signal or pulse will likely interfere with the service signals. Thus, active TDR may not be suitable or practical for a commercial HFC network.
In the scenario where a network must be tested while service signals (or commercial signal traffic) are present in the network (e.g., in a cable TV network, data wire lines in an aircraft or ship, radar, radio intelligence, radio astronomy, non-linear radar, etc.), systems have been developed that use the service signals as a probe signal. For example, see U.S. Pat. No. 7,069,163 to Gunther, where the original data signals on aircraft wires are altered by spread spectrum techniques and then used as a probe signal. The use of existing service signals as the probe signal is referred to herein as a “passive” technique, because no extra (or active) probe signal is introduced. The system proposed in U.S. Pat. No. 7,069,163 to Gunther is not a truly passive technique, however, because it creates a separate probe signal from the original data signals and injects the probe signal into the wire under test (which is also carrying the unaltered original data signals). In the HFC cable television industry, a truly passive technique is used (i.e., a kind of nonlinear radar technology) for detection and location of common path distortion (a nonlinear impairment). This technology is commercially available and known as the Hunter® Xcor system available from Arcom Digital, LLC, Syracuse, N.Y. (http://www.arcomlabs.com/4HunterPlatform.html). The Hunter® Xcor system is described in U.S. Pat. No. 7,415,367 to Williams and in U.S. Pat. No. 7,584,496 to Zinevich (the inventor herein).
A passive technique is also known for the detection of linear (as opposed to nonlinear) impairments in a coaxial cable plant of an HFC network. This technique was developed by CableLabs® as part of the Proactive Network Maintenance (PNM) initiative and is known as InGeNeOs™. It is described in a published document entitled, DOCSIS® Best Practices and Guidelines, Proactive Network Maintenance Using Pre-Equalization, CM-GL-PNMP-V02-110623, by Cable Television Laboratories, Inc., Louisville, Colo., Jun. 23, 2011 (http://www.cablelabs.com/wp-content/uploads/2014/05/CM-GL-PNMP-V02-110623.pdf). The InGeNeOs™ technique is based on using pre-equalization coefficients from cable modems (CMs) operating in the HFC network. It can be classified as a form of passive TDR, because upstream QAM service signals are used as probe signals. Briefly, this technique allows by analysis of pre-equalization coefficients the detection of micro-reflections of an upstream QAM signal on its way from the CM to the CMTS, and a time delay of a reflected signal relative to the initial signal is measured. Then, by using electronic maps a probable point in the network where the micro-reflections were caused (impedance mismatches) is located. The declared accuracy of this technique is about +/−50 feet. Obviously, it is not as accurate as active TDR, and the accuracy is limited by the bandwidth of one upstream QAM channel. Currently, this technology is widely used in cable TV HFC networks.
When a technician is in the field, it would be very helpful for him or her to use a handheld meter to further pinpoint the location of the mismatch in the coaxial plant, and confirm after the work has been done that the problem has been fixed. In using the above-mentioned pre-equalization method, the technician would have to connect to a CMTS and poll data from the CM that has initially raised an alarm of the problem. Obviously, this would provide the most valid confirmation that a problem (mismatch) has been fixed, but it takes extra time, effort and equipment and does not offer the capability of pinpointing the mismatch to less than +/−50 feet. Also, it is limited to the use of upstream signals only. It would be more sensible, from the point of view of locating mismatches in an HFC coaxial plant, to use the higher frequency, broader bandwidth downstream signals to detect linear impairments such mismatches.
CableLabs® has proposed to use the full downstream signal as a passive probe signal. The downstream signal would be captured by modern CMs containing a new Broadcom® chipsets. See, e.g., presentation entitled, Distortion Testing in Home Terminals, CableLabs® SCTE Conference Demo, Oct. 21-24, 2013, by Thomas Williams and Alberto Campos. The idea is to download from the CM a captured full-spectrum downstream signal and then calculate the autocorrelation function of the captured signal combined with an echo of the captured signal. The echo will appear as a spike in addition to the main detection peak in the autocorrelation function. Thus, the time-distance between the main peak and the echo spike can be measured, and this will allow one to measure a time delay of the echo signal more accurately than the approach using pre-equalization coefficients. However, this method has some drawbacks. First, to provide good sensitivity of detection of the echo, the accumulation time during the estimation of the autocorrelation function should be long enough, e.g., tens of milliseconds, while the duration of the captured full downstream signal at the CM (used generally for spectral analysis) is limited to tens of microseconds. The capture duration at the CM is limited so as to achieve a reasonable resolution bandwidth for spectral analysis (e.g., 30 kHz) and a reasonable data size for one capture (e.g., 30 Kbytes). Therefore, to achieve an adequate accumulation time it will be necessary to make a large number of captures. However, this will take time and impact CM signal traffic. Another limitation of this method is that continuous wave (CW) or narrowband downstream signals, such as analog video carriers, AGC pilots, and OFDM continuous pilot subcarriers, may be included in the autocorrelation, which would contribute flat or spread components in the autocorrelation function and may cause an echo spike to be masked. This could reduce sensitivity and time resolution (or distance accuracy).
Obviously, for the scenario of pinpointing linear impairments within the last tens of feet, the technician will not actually have an opportunity to check linear distortion by polling data from nearby CMs. Also, implementing a method that captures the whole downstream signal in a handheld meter for analysis will not be cost effective, because high resolution analog-to-digital converter chips, with sampling rates higher than 2 GHz are needed to capture the downstream signal. The chips are still very expensive and have high power consumption (e.g., a few Watts). Also, to undertake the necessary signal processing of a full downstream spectrum, a powerful, high cost, high power consuming, computer processor would be required.
Neither the pre-equalization coefficients method nor the whole-downstream signal capture method discussed above is actually a classical TDR method, where the time delay of the echo signal is measured relative to a connection point of the TDR meter. Both methods detect only the fact that reflections have occurred between the CM and the fiber node, or vice versa. Therefore, both methods require a complex analysis of data from many CMs and correlation of data with electronic maps. The above methods are definitely useful from the point of view of alerting operators of the presence of linear impairments and identifying a probable zone of locations of the mismatched device(s). However, they are less useful for pinpointing mismatches within the last tens of feet in a field search for mismatches.
Another known method and system for testing a network path while carrying operational (or service) signals is described in U.S. Pat. No. 7,271,596. This patent describes a method of passive TDR based on estimating an autocorrelation function of the full-spectrum operational signal. However, in a modern HFC television network, the capturing and processing of the full-spectrum operational signal and estimating its autocorrelation function is not a trivial task and has certain drawbacks, as discussed above and listed here:
(1) The HFC cable television downstream signal has a very wide bandwidth (e.g., 1 GHz with a probable increase to 1.7 GHz under DOCSIS 3.1). Therefore, sampling and digitally processing this signal requires expensive and high power-consuming chipsets, which make it impractical to implement a passive TRD system in a low-cost handheld unit.
(2) The HFC cable television downstream signal is not completely random and includes many CW pilots which have flat or spread autocorrelation responses. Such flat responses may mask an echo spike in the autocorrelation function. Even with infrastructure migration to all digital signals, the OFDM signal contains a number of continuous pilot subcarriers as part of the OFDM spectrum. The detection of OFDM pilot subcarriers and Fast Fourier Transform (FFT) processing of such signals requires a minimum 1 kHz resolution. This means that if the full downstream spectrum signal is captured, the number of points for the FFT process would be extremely large, like approximately 2 GHz/1 kHz=2×10^6.
(3) TDR is generally used in a section of a coaxial plant that contains only passive network devices (“passive section”), e.g., between adjacent line or trunk amplifiers. Downstream signals passing through an amplifier may already contain reflected signals which could, in an autocorrelation function, mask echo spikes of impairments located in the passive section under test and create false detections (see FIG. 10). Even random signals, which would theoretically produce Dirac autocorrelation functions, may have extra, undesired peaks and sidelobes in their autocorrelation functions after passing through branches of the HFC network with cascaded amplifiers. These extra peaks and sidelobes may interfere with desired measurements.
As a result of the above-discussed problems, it has been a challenge to attempt a low-cost, low power-consuming implementation of a passive TDR system for an HFC cable television network. Accordingly, a need exists for an innovative approach that will make it possible and practical to achieve such an implementation in a portable hand-held unit.